Advances in semiconductor integrated circuit design, processing, and packaging technologies have resulted in increases in the number and density of input/output (I/O) connections on each die and as well as in an increase in the diameter of the silicon wafers used in device fabrication. With increasing numbers of I/O connections per die, the cost of testing each die becomes a greater and greater fraction of the total device cost. The test cost fraction can be reduced by either reducing the time required to test each die or by testing multiple die simultaneously.
Probe cards may be used to test single or multiple die simultaneously at the wafer level prior to singulation and packaging. In multiple die testing applications, the requirements for parallelism between the array of spring probe tips on the probe card and the semiconductor wafer become increasingly stringent since the entire array of spring probe tips are required to make simultaneous electrical contact over large areas of the wafer.
With each new generation of IC technology, the I/O pitch tends to decrease and the I/O density tends to increase. These trends place increasingly stringent requirements on the probe tips and associated probe card structures. Fine pitch probe tips are required to be smaller in width and length while continuing to generate the force required to achieve and maintain good electrical connections with the device under test. The force required to achieve a good electrical connection is a function of the processing history of the IC contact pad, such as but not limited to the manner of deposition, the temperature exposure profile, the metal composition, shape, surface topology, and the finish of the spring probe tip. The required force is also typically a function of the manner in which the probe tip “scrubs” the surface of the contact pad.
As the probe pitch decreases, the linear dimensions of the IC connection terminal contact areas also decreases, leaving less room available for the probe tips to scrub. Additionally, the probes must be constructed to avoid damaging the passivation layer that is sometimes added to protect the underlying IC devices (typically 5-10 microns in thickness). Additionally, as the spring probe density increases, the width and length of the probes tends to decrease and the stress within the probe tends to increase, to generate the force required to make good electrical contact to the IC connection terminal contact areas.
There is a need for probe cards for fine pitch probing comprised of an array of spring probe contacts capable of making simultaneous good electrical connections to multiple devices on a semiconductor wafer under test in commercially available wafer probers using specified overdrive conditions over large areas of a semiconductor wafer and or over an entire wafer. To accomplish this, the array of spring probe contacts on the probe card should be co-planar and parallel to the surface of the semiconductor wafer to within specified tolerances such that using specified overdrive conditions, the first and last probes to touch the wafer will all be in good electrical contact with the IC device yet not be subject to over stressed conditions which could lead to premature failure. Additionally, any changes in the Z position, e.g. due to set or plastic deformation, or condition of the probe tips, e.g. diameter, surface roughness, etc., over the spring probe cycle life should remain within specified acceptable limits when operated within specified conditions of use, such as but not limited to overdrive, temperature range, and/or cleaning procedures.
Micro-fabricated spring contacts are potentially capable of overcoming many of the limitations associated with conventionally fabricated spring contacts, e.g. tungsten needle probes, particularly in fine pitch probing applications over large substrate areas. Micro-fabricated spring contacts can be fabricated using a variety of photolithography based techniques known to those skilled in the art, e.g. Micro-Electro-Mechanical Systems (MEMS) fabrication processes and hybrid processes such as using wire bonds to create spring contact skeletons and MEMs or electroplating processes to form the complete spring contact structure. Arrays of spring contacts can be either be mounted on a contactor substrate by pre-fabricating and transferring them (either sequentially or in mass parallel) to the contactor substrate or by assembling each element of the spring contact array directly on the contactor substrate, such as by using a wire bonder along with subsequent batch mode processes.
Micro-fabricated spring contacts may be fabricated with variety of processes known to those skilled in the art. Exemplary monolithic micro-fabricated spring contacts may comprise stress metal springs having one or more layers of built-in or initial stress that are photolithographically patterned and fabricated on a substrate using batch mode semiconductor manufacturing processes. As a result, the spring contacts are fabricated en masse, and can be fabricated with spacings equal to or less than that of fine pitch semiconductor device electrical connection terminals or with spacings equal to or greater than those of printed circuit boards, i.e. functioning as an electrical signal space transformer.
In exemplary stress metal spring contacts, an internal stress gradient created within the spring contact causes a free portion of the spring contact to bend up and away from the substrate to a lift height that is determined by the magnitude of the stress gradient. An anchor portion remains fixed to the substrate and is electrically connected to a first contact pad on the substrate. The spring contact is made of an elastic material, which provides the free portion of the spring contact with flexibility and mechanical compliance. The force generated by stress metal spring contacts can be increased by the application of one or more plated metal layers comprising metals and metal alloys with appropriate Young's modulii, such as nickel or nickel cobalt, etc. Increasing the spring force helps to establish reliable electrical contacts and can also help to compensate for mechanical variations and induced by temperature changes and other environmental factors.
Photolithographically patterned spring structures are particularly useful in electrical contactor applications where it is desired to provide high density electrical contacts which may extend over relatively large contact areas and which also may exhibit relatively high mechanical compliance in the normal direction relative to the plane of the contact area. Such electrical contactors are useful for applications including integrated circuit device testing (both in wafer and packaged formats), integrated circuit packaging (including singulated device packages, wafer scale packaging, and multiple chip packages) and electrical connectors (including board level, module level, and device level, e.g. sockets. Photolithographically patterned spring structures are also well suited for the fabrication of electrical interposers capable of providing compliant electrical connections between arrays of electrical contacts.
As device pitch decreases and for other reasons mentioned above, it would be desirable for micro fabricated contactors and interposers to possess contact elements having increasing higher levels planarization and/or lift height uniformity.
It would be advantageous to provide a method and structure to fabricate improved microfabricated spring contacts either directly or indirectly across the surface of substrate areas, which can provide increased strength, longevity, and planarity, as well as superior electrical contact performance, over a wide variety of operating conditions e.g. temperature, cycle life, overdrive, pad metal, etc. Such a development would provide a significant technical advance.
Due to the inherent compatibility with the planar manufacturing processes used in printed circuit board and semiconductor fabrication, as well as the high level of inherent complexity, computer automated design or CAD tools are frequently utilized in the design and layout of contactors and probe cards comprising photolithographically patterned microfabricated spring contacts.
While it is sometimes desirable to employ commercially available CAD tools when their use is efficient and cost effective, it is sometimes desirable to develop or utilize customized design and layout tools in order to optimize and/or shorten the design cycle time or to provide functionality not present in commercially available CAD tools. Additionally, the specific design requirements of MEMs and/or photolithographic processes may best be addressed with customized CAD tools.
It would therefore be desirable to provide customized design tools that increase the level of automation and speed of CAD tools utilized in the design and layout of contactors and probe cards comprising photolithographically patterned microfabricated spring contacts. Such improvements would provide significant advances in the efficiency and cost effectiveness of contactor and probe card design.
The level of design complexity and hence the design cycle time is also a function of the physical architecture of the microfabricated device. It would therefore be desirable to reduce the design cycle time by optimizing the physical architecture of the microfabricated system by providing physical architectures that utilize a combination of standard components and customized CAD tools to provide reductions in the design and fabrication cycle time. Such improvements would also provide significant advances in the efficiency and cost effectiveness of contactor and probe card design.